EXAMPLES
For firewalls and routers with multiple interfaces, it is desirable to
failover all of the carp interfaces together, when one of the physical
interfaces goes down. This is achieved by the preempt option. Enable it
on both host A and B:
sysctl net.inet.carp.preempt=1
Assume that host A is the preferred master and 192.168.1.x/24 is config-
ured on one physical interface and 192.168.2.y/24 on another. This is
the setup for host A:
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24
The setup for host B is identical, but it has a higher advskew:
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24
Because of the preempt option, when one of the physical interfaces of
host A fails, advskew is adjusted to 240 on all its carp interfaces.
This will cause host B to preempt on both interfaces instead of just the
failed one.
In order to set up an ARP balanced virtual host, it is necessary to con-
figure one virtual host for each physical host which would respond to ARP
requests and thus handle the traffic. In the following example, two vir-
tual hosts are configured on two hosts to provide balancing and failover
for the IP address 192.168.1.10.
First the carp interfaces on host A are configured. The advskew of 100
on the second virtual host means that its advertisements will be sent out
slightly less frequently.
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24
The configuration for host B is identical, except the advskew is on vir-
tual host 1 rather than virtual host 2.
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24
Finally, the ARP balancing feature must be enabled on both hosts:
sysctl net.inet.carp.arpbalance=1
When the hosts receive an ARP request for 192.168.1.10, the source IP
address of the request is used to compute which virtual host should
answer the request. The host which is master of the selected virtual
host will reply to the request, the other(s) will ignore it.
This way, locally connected systems will receive different ARP replies
and subsequent IP traffic will be balanced among the hosts. If one of
the hosts fails, the other will take over the virtual MAC address, and
begin answering ARP requests on its behalf.
Note: ARP balancing only works on the local network segment. It cannot
balance traffic that crosses a router, because the router itself will
always be balanced to the same virtual host.
SEE ALSO
inet(4), rc.conf(5), ifconfig(8), sysctl(8)
HISTORY
The carp device first appeared in OpenBSD 3.5. The carp device was